1. Field of the Invention
The present invention relates to an apparatus and method for aligning a rifle scope to a rifle, and more particularly toward an apparatus and method in which either the reticle or reticle adjustment axis of the rifle scope can be quickly and accurately aligned to the bore of the rifle.
2. Description of the Prior Art
A rifle scope operates by superimposing an aiming reference mark on an image of the target. The aiming reference mark may be a reticle crosshair (intersection of two orthogonal straight lines), a “dot” or other reference mark either printed on glass or suspended on wires. The aiming reference mark may also be illuminated for use in low ambient light, or it may be formed by emission from a point source of light such as a light emitting diode, optical fiber, or illuminated pinhole. For purposes of this discussion, the aiming reference mark will be referred to as the crosshair. A line that passes through the center of the Earth is referred to as being plumb or vertical, and a line or plane that is orthogonal to a plumb line is referred to as horizontal or level. Two adjacent surfaces that are oriented at 90 degrees +/−1 degree with respect to each other are referred to as being square.
In rifle scopes the crosshair is located at the focus of the objective lens, and therefore the crosshair appears superimposed over the image of the target. The optical axis formed by the crosshair and the center of the objective lens is called the crosshair axis. The lateral position of the crosshair is controlled by use of a micrometer adjustment mechanism. By rotating two micrometers, one controlling the elevation crosshair position, and the other controlling the windage crosshair position, the crosshair axis can be adjusted as needed or desired. These two micrometer adjustment mechanisms move the reticle along axes that are orthogonal to the axis of the rifle scope tube or optical axis, and that are orthogonal to each other. The rifle is typically held such that the elevation turret axis is oriented in approximately the vertical direction and the windage turret axis is oriented in approximately the horizontal direction. The micrometer adjustment mechanism is often called a turret and the part of the turret that is manipulated by the user is often called the turret knob.
Rifle scopes are typically secured to rifles by use of rings that are split in half horizontally, creating a saddle that mounts to the rifle and a clamp that secures the scope tube to the saddle. Alternatively, the rings may be split vertically, creating two halves that clamp to a rail mounted on the rifle, while also clamping around the rifle scope tube. Typically, machine screws are used to secure one half of the ring to the other, clamping the scope tube between the two halves and thereby preventing movement of the scope due to recoil of the rifle or other disturbance. The rifle scope is typically mounted above the rifle bore. The crosshair axis is not parallel to the rifle bore axis, but rather it crosses the bore axis at a distance from the rifle of typically 50-200 yards, as determined by the person setting up the rifle.
The rifle is aimed by moving the rifle while looking at the target through the rifle scope. The direction of the rifle bore is moved until the crosshair coincides with the desired point of impact of the bullet fired from the rifle.
Typically, before the scope is secured in the rings, it is first aligned to the rifle by making a flat reference surface of the scope parallel to a corresponding flat reference surface on the rifle, usually a surface on the rifle receiver. Then the crosshair axis is adjusted to coincide with the point of impact of bullets at a specific range, say 100 yards. This process of live fire crosshair alignment is called “zeroing” the rifle.
To aim at targets at a different range than that at which the rifle was zeroed, the crosshair is raised or lowered by rotating the elevation turret knob to compensate for the effect of gravity on the bullet during flight, which is called bullet drop. The position of the crosshair may also be moved by rotating the windage turret knob to compensate for the effect of crosswind on the bullet trajectory.
Two types of aiming error can result when the rifle is zeroed at one range and then the reticle is adjusted to fire at a target at a much different range. First, if the elevation micrometer axis is not parallel to the direction of gravity, the movement of the crosshair will not be aligned with the direction of bullet drop. This aiming error is typically referred to as a canting error due to the rifle being rotated or “canted” left or right about the bore axis. A spirit level may be attached to the rifle or rifle scope to aid in determining a plumb orientation of the elevation turret axis. Alternatively, the reticle may have reference marks along a vertical line forming the crosshair. The rifle is raised so that one of these marks, corresponding to the target range, is superimposed over the target. In this case, a canting error may occur if the vertical line forming the crosshair is not parallel to the direction of gravity.
A carpenter's level, plumb bob or other leveling device is typically used to set the rifle scope orientation such that the elevation turret axis is plumb to the Earth when the anti-cant indicator is installed. This leveling device is typically placed against a flat reference surface of the rifle scope, such as a side of the turret housing or the end of a turret knob. Alternatively a carpenter's level is used to orient a vertical line drawn on a piece of paper such that the line is plumb. Then the vertical line is viewed through the rifle scope and the scope is rotated in the rings until the vertical line forming the crosshair is parallel to the plumb line. Alternatively, some rifle scope reticles have at least one long prominent horizontal line and only short vertical lines, one of which is prominent and forms the crosshair at the intersection of the prominent horizontal line. In this case, the scope is rotated until the horizontal line forming the crosshair is parallel to a horizontal line, typically printed or drawn on a piece of paper.
The leveling device must contain a spirit level comprised of a tubular bubble vial that is accurately aligned to a horizontal straight edge, and preferably have at least one vertical edge that is perpendicular to the horizontal edge. The tubular bubble vial is oriented with respect to the horizontal straight edge such that the bubble in the vial is centered between symmetric indicator marks when the straight edge is horizontal.
Leveling devices commonly available for leveling a rifle scope typically are mass-produced using injection molded parts that hold a tubular bubble vial securely in the device. This type of construction usually does not produce an accurate spirit level. When placed against a horizontal surface, devices of this type more often than not indicate that the horizontal surface is not level. Errors of greater than +/−1 degree are common in such devices. Usually a precision machined and assembled device is needed to align a surface horizontal to within +/−1 degree. Alternatively, machinist's levels have an adjustable tubular bubble vial that must be calibrated using a horizontal surface plate. Such leveling devices are expensive and heavy, and/or require expertise and special equipment to calibrate, and therefore are not well suited for use by sportsmen, competitors and recreational firearm enthusiasts.
A second aiming error may occur if the rifle scope has a horizontal offset from the rifle bore axis 90 (See FIG. 7A), such that the elevation turret axis 96 or vertical crosshair axis 98 does not pass through the center of the rifle bore. (See FIGS. 7 and 7A) In this case there is a horizontal component 105 to the angle subtended between the crosshair axis 94 and the rifle bore axis 90. This horizontal component
It is the object of the present invention to provide a rifle scope vertical alignment device and method that is simple in construction, inexpensive to produce and contains no moving parts.
It is a further object of the present invention to provide a device and method that allows the rifle scope to be quickly, conveniently and accurately aligned such that the vertical reticle axis or elevation turret axis is projected through the center of the rifle bore and that includes double-sided adhesive tape in order to affix the device to the objective bell.
It is a further object of the present invention to provide an accurate spirit level that can be used to level or plumb a flat surface on the scope by first locating an edge of the device parallel to a flat surface on the rifle scope wherein said spirit level is visible from three sides.
It is a further object of the present invention to provide a device with a center of gravity that allows the device to stand inverted, resting on a flat, approximately horizontal surface.
It is a further object of the present invention to enable a method of manufacturing the device that provides a single machine for production of the substrate, and yet provides a durable reticle alignment device that includes an accurate spirit level and has square edges.